In the current design, this light is due to fluorescent light incident on the R-tips at the solid-liquid interface scattering into the acceptance cone of the receiving fibers or by transmission of light incident on the R-tips at an angle outside of the acceptance cone of the receiving fibers through cladding modes in the fiber.

This light may be comparable in intensity to the directly collected fluorescent light because, as in Fig. 3, the “acceptance cone” of the R-fibers may not overlap the primary illumination zone that emits diffuse fluorescent light in all directions.

Examples of other factors are as follows: photo detector’s area, type, and number of optical filters in the system, opposite wall reflectivity, indices of refraction of the doped liquid and vapor, reflectivity and diffusivity of nearby surfaces, time duration of solution excitement by laser, ambient light (dark room, day, night, etc.), location and size of I and R fibers, etc. Also see “Other terms and definitions” in Sec. 5 for variables such as ELD, T, etc.

To overcome the limitations/disadvantages of many known liquid film thickness sensing devices (viz. conductivity probes, reflectance based fiber-optics probes, capacitance probes, etc.), a new liquid film thickness sensor that utilizes fluorescence phenomena and fiber-optic technology has been developed and reported here. Measurements from this sensor are expected to facilitate better understanding of liquid film dynamics in various adiabatic, evaporating, and condensing film flows. The sensor accurately measures the instantaneous thickness of a dynamically changing liquid film in such a way that the probe does not perturb the flow dynamics in the proximity of the probe’s tip. This is achieved by having the probe’s exposed surface embedded flush with the surface over which the liquid film flows, and by making arrangements for processing the signals associated with the emission and collection of light (in distinctly different wavelength windows) at the probe’s flush surface. Instantaneous film thickness in the range of 0.5–3.0 mm can accurately (with a resolution that is within ±0.09mm over 0.5–1.5 mm range and within ±0.18mm over 1.5–3.0 mm range) be measured by the sensor described in this paper. Although this paper only demonstrates the sensor’s ability for dynamic film thickness measurements carried out for a doped liquid called FC-72 (perfluorohexane or C6F14 from 3M Corporation, Minneapolis, MN), the approach and development/calibration procedure described here can be extended, under similar circumstances, to some other liquid films and other thickness ranges as well.

Scaled schematic of preliminary concept probe. Note that the zones within the primary illumination cone from which fluorescent light is able to enter one or more receiving fibers directly within their acceptance cones (dark regions where the primary illumination cone and receiving fiber acceptance cone(s) intersect).

(a) Errors E1 of the developed sensor for interface angle of ±10 deg; (b) correspondence of estimated error ET for D and error for Sf∣L-V; (c) correspondence of estimated errors between Sf∣L-V and Sf∣No L-V for this calibration; and (d) resolutions of the developed sensor displayed on the Sf∣No L-V versus film thickness curve

Appearance of a nonmonotonic D−Sf∣L-V curve if NP is not sufficiently small with respect to D. This test was done under conditions of a higher LD excitation current. The filtering is described in Ref. 35.

(c) Signal D obtained for dynamic films at a fixed location in the calibration chamber in Fig. 1; (d) signal D obtained for stationary films of the same mean thickness at the same measurement location as in Fig. 1; (a) dynamic film thicknesses measured from the calibrated signal in Fig. 1 and its comparisons with alternative measurements (marked by x); and (b) stationary film thicknesses measured from the calibrated signal in Fig. 1 and its comparisons with alternative measurements (marked by x)

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